Metal and mixed metal oxides are useful as functional materials to provide heat resistance/conduction, as structural materials, composite membranes for fuel cells, solar cells, catalysts, controlled delivery, coatings, light diffusers, cosmetics, ceramic glasses, etc., wherein the particle size and morphology such as shape, internal porosity, and surface area may be important. Included among the metals used in such compounds are tetravalent metals such as zirconium, vanadium, titanium, and silicon, although lower valence metals such as aluminum or yttrium are also particularly useful. Because of its relative abundance, silicon oxides have been the material of choice for a wide variety of commercial usages from nanoparticles used in polymer composites to synthetic silica for production of ultrahigh purity fused quartz materials for semiconductor applications.
Controlled particle size, surface area, and porosity have been a matter of interest in recent years due to wide commercial applications. Current techniques for producing metal oxides, however, have a number of limitations and do not offer a single chemistry for providing a wide range of properties.
More specifically, for silicon metal and its corresponding silicon dioxide (silica), current techniques for synthesis of synthetic silica powder are based on sol-gel methods, ion-exchange of sodium silicate glass, and sol-gel of fumed silica derived from silicon tetrachloride or other oxidizable silicon source i.e. octamethylcyclotetrasiloxane in the presence of oxygen and/or water. Commercially available synthetic silica glass is primarily made utilizing; (1) a method in which a fume generated by decomposition of silicon tetrachloride or an organosilicon material in an oxyhydrogen flame is deposited and grown on a substrate; (2) a method in which a silica gel obtained by hydrolysis and gellation of a silicon alkoxide, is baked and a synthetic silica powder thereby obtained is further densified to make a glass; or (3) direct sol-gel processing usually used for smaller parts.
However, the method (1) has a problem in that the production cost is extremely high. On the other hand, in the method (2) employing a silica gel, particularly a silica gel derived from a silicon alkoxide, it is possible to obtain a synthetic silica powder having relatively low content of minor impurities, but the desired impurity level is not necessarily satisfied. Method 3 requires slow and extensive drying to produce parts without cracking and has not been found economically viable to this point for large shapes.
Silica sol has been conventionally made by using a sodium silicate solution called water glass as a starting material. In this way, sodium silicate solution is treated using a cation-exchange resin so that ions, such as sodium ion in sodium silicate, are removed in order to increase the purity of the starting material. Then, the resultant material is used for producing silica sol. Such a technique is disclosed in U.S. Publication No. 2007/0237701, which is incorporated by reference. However, because the above-method employs an ion-exchange resin for purification, its purity is limited to some degree. Thus, it is difficult to produce silica sol having 1 ppm or lower content of metal impurities, such as alkali metals (e.g., sodium), copper, nickel, and/or aluminum, which is required for use with electronic materials.
Alternatively, methods for making relatively high purity synthetic silica particles include hydrolyzing and condensing alkoxysilanes and using pulverization techniques. An example of a method using pulverization techniques for preparation of silica particles useful for high purity synthetic silica applications is disclosed in U.S. Pat. No. 6,131,409, which is incorporated by reference. Because of restrictions with respect to the purity of the starting material and the many process steps required to achieve the final particle size, various other processes have been attempted.
These methods and techniques, however, also have a number of limitations. For example, they do not appear to produce very high yields of silica. Also, as disclosed in U.S. Pat. No. 4,767,433, these methods and techniques do not form silica having a desired particle size, such as the particle size of 200 to 300 microns required by some fusion processes. These relatively larger particle size ranges also would be useful in the production of crucibles for semiconductor and glass articles useful to semiconductor processing such as, for example, racks, windows and containments, and fiber optics.